1. Field of the Invention
This invention relates in general to oil well valves, and in particular to an annulus valve used below a tubing hanger in a subsea well.
2. Description of the Prior Art
In a producing oil well, a string of tubing will be suspended within casing. The tubing is suspended at the wellhead by a tubing hanger. Normally, a downhole packer will seal the tubing to the casing. The produced fluids flow through the tubing to the surface.
Periodically, the pressure in the annulus between the tubing and casing must be checked to determine if any leakage exists. Also, when workoever operations are being performed, it may be necessary to circulate fluids down the tubing and up the annulus. In a subsea well, the opening and closing of the annulus is preferably handled remotely.
Some tubing hangers for subsea wells have two parallel passages, one for the production fluid and one for the annulus. An external valve will control the annulus in those types. Each passage is offset from the axis of the tubing hanger. In the concentric tubing hanger type, the production fluid passage is centered on the axis of the tubing hanger. A sliding sleeve valve is mounted to the tubing hanger below the seals between the tubing hanger and wellhead. An annulus passage extends through the body of the tubing hanger from the top to a lower cylindrical portion formed on the tubing hanger.
The sliding sleeve is mounted to the lower cylindrical portion. The sliding sleeve has a port that will align with the annulus passage port in the tubing hanger to communicate the annulus to a point above the tubing hanger. Hydraulic pressure applied from the surface will move the sleeve down, and a spring will force the sleeve back up when hydraulic pressure is removed.
While this type of annulus valve is successful, a possibility for a problem exists. Elastomeric seals are located above and below the annulus port on the body of the tubing hanger. The port in the sliding sleeve slides past the lower seal as it moves between the open and closed position. There is a possibility of damage to the lower seal as the sliding sleeve port slides past the lower seal. If such occurred, the tubing hanger may have to be pulled from the wellhead to replace the seal.